1. Field of the Invention
This invention relates to a device for splitting light by use of a diffraction grating. More particularly, this invention is concerned with a device which is adaptable to light metering devices in photographic cameras.
2. Description of the Prior Art
In light metering devices generally used in photographic cameras, an image forming light beam which has passed through an object lens is split by means of a beam splitter to form a portion of light beam which is detected by a photodetector. In the beam splitter useful for such light metering devices, there has not been used a single half mirror which is arranged in an oblique position within the camera. A beam splitting device used in photographic cameras and the like should desirably be as thin as possible.
U.S. Pat. No. 3,464,337 discloses a device for splitting the incident light in a geometrical-optics manner for light metering, in which two pieces of members, each being provided with a plurality of slant reflective surfaces, are cemented to each other to form a prismatic line raster.
The present invention is to provide a beam splitting device which is much more reduced in thickness by using a diffraction grating.
In a beam splitter for obtaining a portion of light to be detected by use of a diffraction, it is desirable to emit a light beam for imaging and a predetermined degree of diffracted light to be metered from a diffraction grating while preventing any unnecessary diffracted light from producing therein. The unnecessary diffracted light results in a flare or ghost which becomes harmful for the image formation.
Japanese Laid-Open Patent Application No. 42042/1978, which has been filed by the applicant, discloses a beam splitting device intended to fulfill the above desire. This beam splitting device utilizes a relief type diffraction grating which can be mass produced effeciently.
As described in the above Japanese Patent Disclosure, the relief type diffraction grating is of a construction having a surface where rigids are periodically found. Such a diffraction grating can be mass produced inexpensively from any stable material known in the art, for example, plastics, by the use of any suitable copying technique such as compression molding.
In the beam splitter utilizing the diffraction grating, a transmissively diffracted light of zero degree is used as an image forming light beam without any modification and also the other transmissively or reflectively diffracted light(s) of No-th or (No.+-.1)-th degrees is (are) conducted in a detector. The beam splitter disclosed in said Japanese Laid-Open Patent Application No. 42042/1978 uses first degree of diffracted light as a portion of light to be metered in addition to zero degree of diffracted light including the image information. As described in the above Japanese Patent Disclosure, this is due to the fact that in the presently known diffraction gratings, it is difficult to increase only two diffracted lights having no adjacent diffractive degree, that is, the zero degree of diffracted light and the second or higher degree of diffracted light in diffraction efficiency.
In general, where it is to split any light in a direction of diffractive angle .theta. by use of a diffraction grating having a pitch d, a relationship between the pitch d and the N-th degree of diffractive angle .theta. can be represented by the following formula: EQU d sin .theta.=N.lambda. (1)
where .lambda. is a wavelength of the incident light.
As can easily be seen from the above formula, the lower the degree N of the diffracted light to be metered is, the smaller the diffractive angle .theta. and, the pitch d become. In order to take the diffracted light to be metered out of the image forming light beam, the diffractive angle .theta. must be above a predetermined angle, for example, in the order of 30 degrees. Where zero degree of transmissively diffracted light and first (N=1) degree of transmissively or reflectively diffracted light are used, respectively, as an image forming light and a diffracted light to be metered, as in the beam splitter disclosed in said Japanese Patent Disclosure No. 42042/1978, the pitch d in a diffraction grating becomes equal to 1.1 .mu.m from the above formula (1) if a diffracted light to be metered having its wavelength .lambda. of 0.55 .mu.m is to be split with a diffractive angle .theta. of 30 degrees. This pitch is very fine so that the diffraction grating must be manufactured with high-level technique resulting in an increased cost of mass production.
A beam splitter overcoming the above problem has been proposed by the applicant as disclosed in Japanese Patent Application No. 10190/1979 corresponding to U.S. patent application Ser. No. 285,921, filed July 23, 1981, now abandoned, which is a continuation of U.S. patent application Ser. No. 114,201, now abandoned. This beam splitter is characterized by a higher degree of reflectively diffracted light utilized for light metering instead of the first degree of transmissively or reflectively diffracted light.
In the prior art relief type diffraction grating before said Japanese Patent Application No. 10190/1979, a lower degree of transmissively diffracted light is simultaneously produced resulting in a harmful ghost formed in the light beam from a view finder as hereinafter described with reference to FIG. 2. In the improved beam splitter disclosed in said Japanese Patent Application, lower degrees of transmissively diffracted light, except the zero degree of transmissively diffracted light, can be avoided by providing a transparent substrate forming a relief type diffraction grating with a ridged surface which is covered by a transparent binder having substantially the same refractive index as that of the transparent substrate.
This technique enables the beam splitter to include its grating portion having an increased pitch of several tens of .mu.m or theoretically larger so that a master used to make the beam splitters can more inexpensively and easily be manufactured. The pitch in the grating portion of the beam splitter has generally been required to be in the order of 1 .mu.m or less.
In reality, however, the binder must be severely selected taking account of various conditions such as optical characteristics including refractive index, transmission factor and others, adhesivity, handling properties and the like. It is not easy to select an appropriate binder from some kinds of binders which can actually be used in this technique for avoiding any harmful light.